1. Field of the Invention
The present invention relates to a discharge electrode of an excimer laser oscillator which is useful for working metals, resins, glass, ceramics, semiconductors, and so forth, and for accelerating chemical reactions. The present invention relates also to a method for shape-restoration of a deformed discharge electrode. In particular, the present invention relates to a discharge electrode which is highly stable in the energy output and the beam shape, and gives a longer life to the excimer laser oscillator, and relates also to a method for shape-restoration of a deformed discharge electrode. Further, the present invention relates to an excimer laser oscillator and a stepper employing the above discharge electrode.
2. Related Background Art
The excimer laser is attracting attention as the high-output laser capable for producing oscillation in the ultraviolet region, and has promising applications in the electronic industry, chemical industry, and energy industry.
The excimer laser oscillator, which produces excimer laser radiation, excites a laser gas such as Ar, Kr, Xe, KrF, and ArF filled in a manifold by electron beam irradiation or electric discharge. The resulting excited atom combines with a ground-state atom to form a molecule which can exist only in an excited state. This molecule, called excimer, is unstable, and immediately emits ultraviolet light to return to the ground state. This transition is called bond-free transition. The excimer laser oscillator multiplies the ultraviolet light produced by the transition by means of an optical resonator constituted of a pair of mirrors or the like, and emits the energy of the ultraviolet light as a laser beam.
An excimer laser system is explained by reference to FIG. 1.
In FIG. 1, the excimer laser system comprises a laser chamber 1, a band-narrowing module 2 to control the spectrum breadth of the oscillated light to be narrow, a pulse power module 3 for applying high voltage to electrodes in the chamber 1, an optical monitor module 4 for measuring the energy intensity and wavelength of the produced laser oscillation, a high voltage power source 5, a controller 6 for controlling the entire excimer laser system, a reflection mirror 7 constituting a resonator, a half mirror 8 for monitoring, a motor 9, and a window 10.
For use of the above excimer laser system as an exposure light source for a stepper, the controller 6 comprising a computer and other devices is connected through an interface to a stepper-controlling unit 12, a stepper-controlling computer 13, and so forth.
The laser chamber 1 is filled with a laser gas, such as F.sub.2, Kr, and Ne. With application of a high-voltage DC pulse from the pulse power module 3 between the cathode electrode and the anode electrode in the laser chamber, electric discharge occurs through the gas, and light is generated. The generated light is reflected repeatedly in the optical system between the output mirror 7 and the band-narrowing module 2 to be amplified, and emitted to the outside.
To obtain a laser beam of an intended wavelength, a part of the emitted laser light is introduced by a half-mirror 8 to an optical monitor module 4 to measure the wavelength of the light. In accordance with the measurement result, a stepping motor 9 is driven to optimize the optical system of the band-narrowing module 2. The power of the laser light is also monitored continually by this optical monitor module.
FIG. 2 is a schematic sectional view of the laser chamber 1 taken vertically to the laser beam oscillation direction.
In FIG. 2, the laser chamber 1 is constructed from two aluminum housing members 21, 22, and is sealed with an O-ring 23. In the chamber 1, a cathode 26 is fixed by an insulator 24 and a cathode-supporting member 25, and an anode 27 is fixed by an anode-supporting member 28 to the housing member. The numeral 29 denotes a sealing member. A connector 30 connects the cathode 26 to a pulse power module 3. The numeral 31 denotes a blower; 32, a heat exchanger; 33, an opening having a mesh filter for gas feed and evacuation; and 34, a dust catcher.
FIG. 3 shows a gas-feeding system which was developed by the inventors of the present invention. The system has three gas-feeding line systems consisting of a 1% Kr/Ne gas-feeding line 41, a 1% F.sub.2 /1% Kr/Ne gas-feeding line 42, and a He gas-feeding line 43. The respective gas-feeding lines have an orifice 44, 45, or 46, and a valve 47, 48, or 49. The valves are connected to a manifold 50. A 100% He gas is used as a purge gas when the window plate of the laser chamber is exchanged, or in other cases.
A connecting tube 51 for gas feeding has a chamber valve 52 and a valve 53, and is connected to the manifold 50. An F.sub.2 gas-replenishing tube 54 is connected to the manifold 50 and to the connecting tube 51 through an injection valve 55 and a flow control orifice 56.
When the pressure in the manifold 50 rises abnormally to exceed a predetermined level, a spring valve 57 opens to release the pressure. The gas-feeding system comprises further an evacuation line 58 for the manifold, a valve 59 for evacuating the interior of the laser chamber when an abnormal phenomenon occurs in the laser chamber and being handled manually similarly as the valve 53, an F.sub.2 -detoxifying device 60, an evacuation pump 61, an evacuation valve 62 provided on an evacuation line for evacuating the manifold 50, a variable valve 63, and a pressure gauge p.
Conventionally, to fill a laser discharge gas of a predetermined F.sub.2 -gas concentration of, for example, 0.1% in the laser chamber, firstly a 1% F.sub.2 /1% Kr/Ne gas is introduced through the manifold 50 and the tube 51 into the chamber 1 to a pressure of 30 kPa, for example, by monitoring with a pressure gauge, and then 1% Kr/Ne gas is introduced therein to a pressure of 300 kPa. Thus the gas mixture filled in the chamber contains 0.1% F.sub.2.
Since the F.sub.2 concentration decreases in the course of repeated laser oscillation, a necessary amount of the 1% F.sub.2 /1% Kr/Ne gas is replenished through the manifold 50 and the tube 54 to the chamber 1 every time.
Conventionally, to avoid fluorination of the electrodes, an alloy having relatively high resistance to fluorine is used as the material for the anode 27 and the cathode 26. Examples of the material include platinum, gold, rhodium, ruthenium, osmium, and iridium as shown in Japanese Patent Application Laid-Open No. 6-152013.
The excimer laser oscillator employing such electrodes involves problems as follows: (1) the discharge is unstable, and the pulse energy varies in a range of up to about 10%; and (2) the electrodes are deteriorated and deformed to render the electric field nonuniform, causing pulse energy variation, and necessitating frequent exchange of the electrodes.